In a prior art knob for a door lock, the knob and a spindle are integrally connected so that when the knob is forcibly twisted off, the spindle is also twisted off at the same time, resulting in possible improper unlocking. In the past, there is proposed an auxiliary lock for the exclusive use of prevention of crimes disposed independently of the knob lock in order to prevent the destruction of the aforesaid auxiliary lock from the outside. However, since the usability of the knob is pursued in terms of its primary object, it has never been proposed to provide an arrangement in which a knob itself is provided with a destruction preventive mechanism. This is because of the fact that the provision of the destruction preventive mechanism on the knob itself results in a lowering of the usability thereof.
The present invention is to prevent the destruction of the knob from the outside without lowering the usability of the knob.
The present invention provides a door lock in which locking is made possible from the indoor side by depressing a push button on the indoor side whereas unlocking is made possible from the outdoor side upon insertion of a key into a cylinder encased in the knob on the outdoor side, the door lock being constructed such that a few protrusions are provided on a foremost end of a knob shaft adapted to transmit rotation of the knob for the door lock to a spindle, said protrusions being fixed to a knob back plate superposed integral with the knob, whereby when an excessive twisting load is applied to the knob, the protrusions are twisted off to provide a disengagement between the knob and the knob shaft so that the knob runs idle. Accordingly, the present invention possesses an advantage that the spindle cannot be twisted off from the outside.
Further, another object of the present invention resides in the following. That is, in the event that the protrusions at the foremost end of the knob shaft are intended to be twisted off to pull out the knob, the knob shaft is locked to the outside spindle preencased in the knob. Accordingly, in the event the aforementioned protrusions on the knob shaft are twisted off, the knob becomes slidable along the knob shaft since the knob shaft is locked to the spindle.
In accordance with the present invention, a knob bottom plate is arranged within a knob hollow portion so as to be loosely slipped on the knob shaft so that when the knob is intended to be pulled out, the knob bottom plate impinges upon the rear wall surface of the knob hollow portion to prevent disengagement of the knob. As such, the present invention has the advantage such that even if an attempt is made to twist the knob or pull out thereof, the knob cannot positively disengaged from the lock mechanism.